The steel strapping machine without using joint has advantages of easy operation, nice appearance and low cost. However, there are three shortcomings described as below limiting the promotion and application thereof. Firstly, the lower mold of the steel strapping machine without using joint is formed as an inseparatable whole which contains a hook, and the hook is used to lift the steel strap before the lower mold is taken out upon a joint is made, so that the lower mold can be taken out. Since the lower mold has a large size along the length of steel strap and has a large thickness, and meanwhile the hook stretches upwards the steel strap, a large gap is generated between the steel strap and a top surface of the strapped package after the lower mold is taken out from underside of the steel strap, resulting in that the strap for the package is not tight enough. As the package to be strapped is smaller, the above-mentioned problem is severer. Secondly, after a joint is made for the steel strap, a situation usually occurs in which the steel strap gets caught on the lower mold when the lower mold is taken out. Thirdly, the lower mold of the steel strapping machine cannot strap small packages due to its design, since as the strapped package is smaller, it is more difficult for the lower mold to be taken out from the underside of the strapped package, and sometimes the lower mold cannot be taken out. Therefore, it is necessary to design a lower mold of the strapping machine, which can strap the packages tightly and can strap small packages, while not catching the steel strap when being taken out.